1. Field of the Invention
The present invention relates to a solid-state image sensor.
2. Description of the Related Art
The basic arrangement of a solid-state image sensor 1 will be described with reference to FIG. 1. The solid-state image sensor 1 comprises a pixel array 10 including a plurality of pixel units 11, and a signal processing circuit 20 for amplifying a signal output from the pixel array 10 to a column signal line 2. In each pixel unit 11, a transfer transistor TX transfers, for example, charges generated by the energy of light received by a photodiode PD to the gate of a source follower transistor SF. Then, the source follower transistor SF outputs a signal corresponding to the transferred charges to the column signal line 2 via a selection transistor SEL. Each pixel unit 11 can include a reset transistor RES for resetting the potential of the gate of the source follower transistor SF to a predetermined voltage.
The signal processing circuit 20 is used to amplify the signal output from each pixel unit 11 to the column signal line 2. The signal processing circuit 20 includes an operation amplifier 21 having an input terminal 22 and an output terminal 23, an input capacitance C0 inserted between the input terminal 22 and the column signal line 2, and a feedback capacitor Cf connecting the input terminal 22 to the output terminal 23. The amplification factor of the signal processing circuit 20 is determined based on a capacitance ratio C0/Cf. To change the amplification factor, for example, to switch the sensitivity setting of a digital still camera, it is only necessary to change the ratio C0/Cf. To change the amplification factor from, for example, 32 to 128, it is only necessary to increase the value of C0 by a factor of four or decrease the value of Cf to a quarter.
To improve the sensitivity of light detection, a high amplification factor is required for the signal processing circuit 20. This especially applies to a case in which a signal output to the column signal line 2 becomes very weak due to the reduction in the size of a photoelectric conversion element such as the photodiode PD or a case in which the sensitivity of a camera having the solid-state image sensor 1 is set to be higher. To increase the amplification factor of the signal processing circuit 20, it is generally necessary (1) to increase the value of C0, (2) to decrease the value of Cf, or (3) to increase the value of C0 and decrease the value of Cf. However, increasing the capacitance value may cause an increase in chip area, and decreasing the capacitance value may cause a production variation.